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authorPaul E. McKenney <paulmck@us.ibm.com>2006-02-01 03:06:42 -0800
committerLinus Torvalds <torvalds@g5.osdl.org>2006-02-01 08:53:25 -0800
commitd19720a909b4443f78cbb03f4f090180e143ad9d (patch)
tree56e579612d82f4b30d5cb943df1079b0b5f4700a /Documentation/RCU/whatisRCU.txt
parent53d8be5c144ece5d48745810b14248968e73eaf2 (diff)
downloadlinux-linaro-android-d19720a909b4443f78cbb03f4f090180e143ad9d.tar.gz
[PATCH] RCU documentation fixes (January 2006 update)
Updates to in-tree RCU documentation based on comments over the past few months. Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'Documentation/RCU/whatisRCU.txt')
-rw-r--r--Documentation/RCU/whatisRCU.txt29
1 files changed, 17 insertions, 12 deletions
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 15da16861fa..5ed85af8878 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -200,10 +200,11 @@ rcu_assign_pointer()
the new value, and also executes any memory-barrier instructions
required for a given CPU architecture.
- Perhaps more important, it serves to document which pointers
- are protected by RCU. That said, rcu_assign_pointer() is most
- frequently used indirectly, via the _rcu list-manipulation
- primitives such as list_add_rcu().
+ Perhaps just as important, it serves to document (1) which
+ pointers are protected by RCU and (2) the point at which a
+ given structure becomes accessible to other CPUs. That said,
+ rcu_assign_pointer() is most frequently used indirectly, via
+ the _rcu list-manipulation primitives such as list_add_rcu().
rcu_dereference()
@@ -258,9 +259,11 @@ rcu_dereference()
locking.
As with rcu_assign_pointer(), an important function of
- rcu_dereference() is to document which pointers are protected
- by RCU. And, again like rcu_assign_pointer(), rcu_dereference()
- is typically used indirectly, via the _rcu list-manipulation
+ rcu_dereference() is to document which pointers are protected by
+ RCU, in particular, flagging a pointer that is subject to changing
+ at any time, including immediately after the rcu_dereference().
+ And, again like rcu_assign_pointer(), rcu_dereference() is
+ typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu().
The following diagram shows how each API communicates among the
@@ -327,7 +330,7 @@ for specialized uses, but are relatively uncommon.
3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
This section shows a simple use of the core RCU API to protect a
-global pointer to a dynamically allocated structure. More typical
+global pointer to a dynamically allocated structure. More-typical
uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
struct foo {
@@ -410,6 +413,8 @@ o Use synchronize_rcu() -after- removing a data element from an
data item.
See checklist.txt for additional rules to follow when using RCU.
+And again, more-typical uses of RCU may be found in listRCU.txt,
+arrayRCU.txt, and NMI-RCU.txt.
4. WHAT IF MY UPDATING THREAD CANNOT BLOCK?
@@ -513,7 +518,7 @@ production-quality implementation, and see:
for papers describing the Linux kernel RCU implementation. The OLS'01
and OLS'02 papers are a good introduction, and the dissertation provides
-more details on the current implementation.
+more details on the current implementation as of early 2004.
5A. "TOY" IMPLEMENTATION #1: LOCKING
@@ -768,7 +773,6 @@ RCU pointer/list traversal:
rcu_dereference
list_for_each_rcu (to be deprecated in favor of
list_for_each_entry_rcu)
- list_for_each_safe_rcu (deprecated, not used)
list_for_each_entry_rcu
list_for_each_continue_rcu (to be deprecated in favor of new
list_for_each_entry_continue_rcu)
@@ -807,7 +811,8 @@ Quick Quiz #1: Why is this argument naive? How could a deadlock
Answer: Consider the following sequence of events:
1. CPU 0 acquires some unrelated lock, call it
- "problematic_lock".
+ "problematic_lock", disabling irq via
+ spin_lock_irqsave().
2. CPU 1 enters synchronize_rcu(), write-acquiring
rcu_gp_mutex.
@@ -894,7 +899,7 @@ Answer: Just as PREEMPT_RT permits preemption of spinlock
ACKNOWLEDGEMENTS
My thanks to the people who helped make this human-readable, including
-Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood.
+Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
For more information, see http://www.rdrop.com/users/paulmck/RCU.